We have begun a series of experiments to measure in vitro transformation and mutagenesis simultaneously in cultured mammalian cells exposed to mutagens and carcinogens. Thus far we have been able to develop a plaque assay which consists of treating a subclone of this line with carcinogens and mutagens and plating them directly into soft agar. The major advantage of this technique (which is only possible with certain types of cultured cells) is that one may measure transformation frequency directly without first plating in liquid medium, retrypsinizing and then plating in soft agar. Thus quantitation is much more accurate. To date we have been able to compare the transformational effects of three carcinogens and ultraviolet radiation. We are still developing a mutagenesis system which we can use in conjuction with the transformation assay; most likely this will be a variation of hypoxanthine-guanine phosphoribosyl transferase system. Within the next six months we should be able to make quantitative comparisons between transformation in vitro and mutagenesis.